Polyurethane coating compositions prepared from blocked polyurethane compounds are disclosed in numerous patents including U.S. Pat. Nos. 3,583,943; 3,846,378; 3,248,371; 3,267,078; 3,694,389; 3,711,571; 4,046,744; 3,993,849; 3,857,818; 3,621,000; and 3,926,875. However, all of the polyurethane coating compositions of the prior art suffer from at least one of the following defects: They are moisture unstable and thus have poor shelf stability and short coating lives; they are required to have solvent contents of greater than about 40 percent; they cure to coatings which are unstable at temperatures greater than about 100.degree. C. and/or are inflexible at temperatures below about 0.degree. C.; or they produce cured coatings which are inelastic, have poor tensile strengths, or are lacking in surface hardness. Additionally, another serious problem encountered with many of the cured polyurethane coatings provided by the prior art is that they have poor weatherability and yellow when used in exterior environments.
The coatings prepared according to the method of the present invention, from blocked isocyanate-functional polymeric compounds and insoluble crosslinking agents, preferably, crystalline polyols, have improved physical and chemical properties over the polyurethane coatings of the prior art. Such properties include high temperature stability, high tensile strength, impact resistance, resistance to degradation and abrasion upon exposure to exterior environments, and reduced tendency to yellow in exterior applications.
Crystalline polyols have heretofore not been useful in preparing cured polyurethane films since they are substantially insoluble in aprotic solvents, which are necessary solvent systems for urethane manufacture. For example, tris(2-hydroxyethyl)isocyanurate, a preferred crystalline polyol crosslinking agent, has been used as a crosslinker for urethane foams (U.S. Pat. No. 4,031,040), but has not been used to any extent in urethane films or compositions due to its very limited solubility in aprotic solvents.
While J. E. Pregler in "Flexible Urethane Resins and Coating Materials Development", Contract No. N0019-700-C-0309, Naval Air Systems Command, Department of the Navy (March 1971) disclosed the use of tris(2-hydroxyethyl)isocyanurate as a crosslinker for high performance urethane coatings, the systems disclosed were limited to one-part moisture curing formulations which have the disadvantage of poor shelf stability, short coating lives and limited curing conditions. Additionally, since tris(2-hydroxyethyl)isocyanurate has such limited solubility in common urethane solvents, a complex solvent system had to be devised. Moreover, the insolubility of the tris(2-hydroxyethyl)isocyanurate limited the solids levels of these compositions to less than 60 percent by weight.
The method of the present invention overcomes the problem of the limited solubility of crystalline polyol crosslinkers in aprotic solvents and allows these crosslinkers to be used in the synthesis of coating compositions, including polyurethane coating compositions.